Place the butter, brown sugar, vanilla extract, rum extract or rum, lemon juice, and cinnamon into a slow cooker. Stir the ingredients together.
Peel the bananas, slice them widthwise into 1-inch discs, and place the pieces in a medium-size bowl with the cornstarch. Carefully toss the bananas until they're coated with the corn starch and spoon them into the slow cooker. Carefully fold them into the butter/sugar mixture until well coated. Set the slow cooker on high and cook for 1 hour.
After the cooking time ends, remove the lid and stir the bananas, being careful not to break them up. Sprinkle the chocolate chips and walnuts over top. Place the lid back on and let it sit for 5 more minutes, or until the chocolate chips are melted. Serve over ice cream.